The aviation industry in the U.S., Canada, Europe and other parts of the world currently utilize an aircraft location system known as the Air Traffic Control Radar Beacon System (ATCRBS). The system consists of an aircraft installed electronic device known as a transponder. The transponder is interrogated from the ground by a dedicated ATCRBS transmitter which is typically associated with a radar system of a ground station or airport. Upon receipt of a valid interrogation signal from the transmitter, the transponder on the aircraft sends back a reply. This operation occurs at a fairly rapid rate so that the information in the reply is instantaneous and continuous. A rotating antenna is used at the ground station so as to be able to determine the azimuth of the reply received from the aircraft transponder, and therefore accurately determine the direction of the aircraft relative to the ground station. By measuring the time between interrogation and reply, the speed of electricity being a known quantity, the ground station can also accurately determine the distance of the aircraft. In addition, the aircraft transponder is connected to a barometric device known as an encoding altimeter. This device automatically (when energized by the pilot) transmits, along with the transponder reply, the altitude of the aircraft.
All of the foregoing information is displayed on a conventional radar screen where an air traffic controller may see the bearing (azimuth), distance and altitude of the aircraft at all times. The need for this system became apparent because of the disparity of radar system target displays, whose d-definition is directly related to the size of the aircraft and the material from which it is manufactured. Larger aircraft will usually provide stronger radar displays than smaller aircraft, which in some cases may hardly be seen at all on the radar system. Fabric and synthetically constructed aircraft also do not provide good radar returns. With transponders in use, all radar displays are equalized, thus allowing for positive management of the aircraft by the air traffic controllers.
The transponder reply from the aircraft has an additional feature of being specifically coded for individual aircraft identification. There are presently 4096 of these codes now in use with the ATCRBS. A specific code is uniquely assigned to each aircraft at the beginning of each flight, and is retained until that flight reaches its destination, thus enabling the orderly transition of the aircraft from one controller and radar system to another as the aircraft continues along its route. There are several specific code designations which are reserved for special usage. The Code 1200 is used by aircraft not under positive control, flying under VFR conditions. This enables the aircraft to be seen by traffic controllers who are required to observe all aircraft within their range to maintain aircraft&. separation required for safe flight. The Code 0000 is used by the U.S. military and is never used by non-military aircraft. The Code 7700 is designated to advise air traffic controllers when an emergency exists (no other method of communication being available). The Code 7600 is used to designate a radio communication failure with all other aspects of the flight being normal. The Code 7500 is used to advise the air traffic controller (without needing to resort to voice communication) that the aircraft is being hijacked.
In recent years the theft of general aviation aircraft has risen dramatically. Surreptitious use of aircraft for smuggling drugs appears to be the leading cause of aircraft theft today. Unfortunately, it is not very difficult to steal an airplane. Many aircraft are parked in unattended unsecured areas which are literally deserted at night. Forced entry to an aircraft is relatively easy. Once inside the cabin, an experienced pilot can start most aircraft without too much difficulty as many cabin class aircraft do not even have ignition key locks. The thief has two choices when he steals an aircraft. The aircraft may either be flown in a clandestine manner without communicating with air traffic controllers and hope that the flight and subsequent landing will go undetected. The other choice is to fly the aircraft in a normal manner communicating with all the many facilities required to utilize the system, and thereby try not to arouse any suspicion.